Electrophotographic marking is a well-known and commonly used method of copying or printing original documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto the latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a receiving substrate such as a sheet of paper. The transferred toner image is then fused to the receiving substrate. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
Most electrophotographic marking machines automatically fed various substrates, typically paper documents, through the machine. For example, blank sheets of paper or documents to be copied are automatically fed through the machine. To assist the marking process it is important that the positions of those moving substrates be tracked as they are fed. One way of tracking substrate positions is to use optical sensors.
Optical sensors provide electrical signals that depend upon the presence, absence, arrival, and/or departure of substrates at a sensed position. Other optical sensors provide information regarding the existence and position of marks on the substrate. FIG. 1 shows a schematic diagram of a typical prior art substrate sensor system 12. As shown, a light emitting diode (LED) 54 emits light at a particular location. A phototransistor 56 is positioned to receive light from the LED 54 when a substrate is not between the LED 54 and the phototransistor 56. When a substrate 58 comes between the LED 54 and the phototransistor 56 the light from the LED 54 is blocked from reaching the phototransistor 56. When light is received by the phototransistor 56 a relatively high electrical signal V.sub.1 is applied to a Schmitt trigger 60. When light is not received by the phototransistor 56 a relatively low electrical signal is applied to the Schmitt trigger 60. The Schmitt trigger then outputs a signal V.sub.0 with one state when a high electrical signal V.sub.1 is present and the other state if a high electrical signal is not present. The signal V.sub.0 thus signifies either the presence or the absence of a substrate 58 between the LED 54 and phototransistor 56. Additionally, since the signal V.sub.0 makes a transition from one state to the other when either a leading edge or a trailing edge of the substrate 58 passes the sensing location, the state transition of V.sub.0 signals the arrival or departure of a substrate at the sending position.
While the substrate sensor system 12 is generally successful, it has a limited sensitivity to document position. This is because the LED 54 emits light over a rather large area (thus is not a point source), because the phototransistor is sensitive to light over a relatively large area (thus is not a point sink), and because the document's position in the z-direction (the substrate 58 is in the x-y plane) is not well controlled. Therefore, when using the substrate sensor system 12 the absolute position of the leading and trailing edges are not accurately known. While position errors may only be tens of microns, in modern high quality electrophotographic printers such errors can be significant.
One way of improving the substrate sensor system 12 would be to incorporate a simple imaging lens between the substrate and phototransistor, forming an image of the substrate onto an image plane located at the phototransistor. The phototransistor could also be replaced with an image sensor. This has the advantage that the magnification can be adjusted by selecting the lens focal length and the front and back conjugates. This is desirable since it may be advantageous to operate with a different size image on the sensor than is present in the paper path. However, with a simple lens as the substrate's position changes in the z direction the magnification varies. This magnification variation changes the size of the image, and hence the apparent position of the document's leading edge, just what is trying to be measured. Therefore, a substrate sensor system that is substantially tolerant of z direction variations would be beneficial.